The present invention relates to an improved shaft locking device which may be used to lock the shaft of a valve in one position and when it is subjected to fire or other excessive heat it releases the shaft to allow it to move to a safe position, for example, to a position in which the gate of the valve is closed. Further, this invention can have application to other shafts which are desired to be held in one position during normal operations and can be released from the locked position either manually or in response to fire or excessive heat.
In U.S. Pat. No. 4,214,600 it is suggested to support the bearings of a gate valve stem with a bushing of heat responsive material, such as eutectic material, which when subject to the heat releases the bearing support so that the stem moves outwardly to a position in which a shoulder on the stem back seats against a shoulder on the bonnet or body surrounding the stem to close against leakage out of the valve around the stem.
It is further known to have a shaft with a groove in its exterior surface and a sleeve surrounding the shaft having ports extending therethrough with a tubular actuator surrounding the sleeve to engage balls in the ports and hold them against the groove in the shaft and partially in the ports so that the sleeve and shaft are locked against axial movement with respect to each other.